The use of adaptive antenna systems is very widespread.
As is known, an adaptive antenna system is generally capable of dynamically altering its radiation characteristics in response to a variation in the characteristics of the channel for receiving and/or transmitting electromagnetic waves.
The characteristics of the reception and/or transmission channel, in turn, mainly depend on the type of device connected to the adaptive antenna system by means of the communication channel itself.
A known type of adaptive antenna systems is represented by reconfigurable antenna structures.
These devices are able to change the orientation of the radiation pattern lobes and/or the polarization of the radiated electromagnetic field by appropriately varying the spatial distribution of the antenna current flowing along the antenna structure.
Traditionally, a reconfigurable antenna structure comprises an active radiating element, electrically connected to a radio frequency source and/or receiving device.
In some known reconfigurable antenna structures, embedded switches or variable capacitors are arranged to change the current distribution along the active radiating element.
In other known reconfigurable antenna structures, passive radiating elements are operationally associated with the active radiating element.
According to some known solutions, the passive radiating elements can be electrically connected/disconnected with a ground plane by means of switching devices.
By operating said switching devices, the passive radiating elements can be short-circuited to ground, thereby varying their electrical length. In this way, they can operate as directors or reflectors of the electromagnetic radiation emitted/received by the active radiating element and vary the radiation characteristics of the antenna structure.
Known reconfigurable antenna structures of this type have some drawbacks.
When the ground plane of the antenna structure is relatively small and/or the distances between the active radiating element, the passive radiating elements and the ground plane are relatively short (as it often occurs in antenna structures having a planar geometry and realized by means of printed circuit manufacturing techniques), current coupling between the active and passive radiating elements is determined by the received/emitted electromagnetic radiation as well as by the currents flowing along the ground plane, which are conveyed by said received/emitted electromagnetic radiation.
When induced currents flow along the ground plane, the ground plane structure emits an electromagnetic radiation by itself, which can sum up in amplitude and phase with the radiation emitted/received by the active radiating element.
This contribution in emitted/received electromagnetic radiation causes a tilt of the radiation pattern of the antenna structure along the plane containing the active/passive radiating elements in their entire length (perpendicular to the azimuth plane).
Experimental tests have proven how said undesired tilt of the radiation pattern may reach also 30° in elevation. This leads to lower gain values and poor coverage along the azimuth plane, since the directivity is enhanced in an unwanted direction.
The performances of the antenna structure may thus remarkably decrease to unacceptable levels, in particular when it is integrated in electronic devices for point-to-point communications, such as access points, gateways, routers, and the like.
The main aim of the present invention is to provide a reconfigurable antenna structure that allows overcoming the aforesaid drawbacks.
A further object of the present invention is to provide an antenna structure that can offer high performance in terms of the configurability of its radiating characteristics along the azimuth plane and the elevation plane.
A further object of the present invention is to provide an antenna structure that can ensure excellent impedance adaptation to the reception and/or transmission channel, as its radiating characteristics vary.
Yet another object of the present invention is to provide an antenna structure that is easy to produce industrially, with relatively low costs, particularly when constructive geometries are adopted, in which the overall dimensions are significantly smaller than the operating wavelengths.